I have passed 1Z0-813 - OCPJP8 upgrade exam today with 81%.
I'm a experienced Java developer with the SCJP certificate. I prepared 2 months part time for this.
Got lots of helpful information from this forum. I would like to provide some feedback for those who are preparing the exam.
Java 8 OCP Study Guide is very good, I read the chapters related to the exam topics, especially Appendix C multiple times.
Enthuware mock exams help a lot. The question style and coverage simulate the real exam, giving you the real exam experience. It also explains each question clearly.
I got average 70% on the mock exams. When I took the real exam, I felt it tougher than the mock exams, but somehow I got better score.
I got lots of questions on stream and lambda expression. My suggestion is to read through those chapters as well as API documentations so you won't be confused during the exam.